The main objective of the research proposed is to study postreplication repair processes during the initiation and promotion of hepatocarcinogenesis in vivo. Rats will be treated with either diethylnitrosamine or 2-acetylaminofluorene at various doses and times relative to enhanced liver DNA synthesis caused by surgical partial hepatectomy. At the time of enhanced DNA synthesis, postreplication repair will be measured in liver slices or hepatocyte cell suspensions using two techniques. The first technique will employ alkaline sucrose gradient centrifugation to detect discontinuities (gaps) and their repair in newly synthesize DNA. The second technique will employ UV irradiated herpes virus as a probe for the presence of error-prone repair functions. The extent of induction of these postreplication repair processes (which may or may not represent the same process) will be correlated with carcinogen dose and amount of excision-repair occurring between carcinogen treatment and induced DNA synthesis. We will also attempt to correlate the extent of postreplication and error-prone repair induction with the appearance of histochemically altered presumptive preneoplastic lesions using short term animal studies. A secondary objective of the research proposed is to continue work on the development of a cell culture model for hepatocarcinogenesis. Specifically, effort will be concentrated on the stimulation of DNA synthesis and cell replication in rat hepatocytes in primary monolayer culture.